There are many research, medical, and military applications for intermediate velocity charged particles, i.e., particles with velocities corresponding to proton energies in the 20-200 MeV range. One particular example is a proton beam for cancer therapy. At present, one conventional technique for accelerating charged particles to the desired energy range is to take the output from a 45 MeV proton beam from a cyclotron and input the beam to a synchrotron for acceleration above 100 MeV. Synchrotrons are relatively complex and expensive machines, however, and it would be desirable to use simpler linear accelerators.
Coupled-cavity and drift-tube linear accelerators are not equally efficient for accelerating particles over an entire energy range of about 20 MeV to 200 MeV. Traditionally, a drift-tube linac (DTL) is the structure of choice for low velocity charged particles in the velocity range around .beta.=0.2 (which corresponds to a 20 MeV proton), where .beta. conventionally represents the ratio of the particle velocity to the speed of light. In this velocity range, the DTL is more efficient than .pi.-mode structures, such as a coupled-cavity linac (CCL), where efficiency is characterized by the effective shunt impedance per unit length (Mohm/m).
But a DTL is a very difficult device to properly tune unless the drift tubes are tightly coupled, i.e., a small number of drift tubes are used. Further, at higher particle velocities, the DTL drops in efficiency because the drift tubes must become longer as particle velocity increases. In addition, DTLs ordinarily require post couplers, i.e., resonant stabilizing devices, to enhance overall beam stability. Post couplers are difficult to model with computer simulations and design optimization generally requires operating prototypes or adjustable hardware that can be optimized in place.
At these low and intermediate velocities, a CCL requires a large number of accelerating cavities, each with a relatively large ratio of cavity surface area to cavity volume, with a concomitant low effective shunt impedance per unit length and low efficiency. At velocities above about .beta.=0.42 (100 MeV proton), the CCL becomes more efficient than the 0-mode DTL. But neither the DTL nor the CCL is efficient over the energy range of 20-200 MeV.
The present invention addresses this problem and combines features of the DTL and CCL to provide a linac over the energy range of 20-200 MeV. Accordingly, it is an object of the present invention to efficiently accelerate charged particles over an intermediate velocity range of 20-200 MeV.
It is another object of this invention to provide a linac with a relatively high shunt impedance per unit length for accelerating intermediate velocity charged particles.
One other object of the present invention is to provide a linac where it is relatively easy to balance the power distribution along the accelerator.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.